Method of producing polyamide membranes and rigid foamed products

ABSTRACT

Polyamide membranes of desired thickness are prepared by dissolving polyamide in HCl, admixing paraformaldehyde and then zinc chloride to produce a viscous paste, forming expanded gelatinous material by treatment with formaldehyde, curing this material and applying tensile stress thereto. Rigid, water insoluble, polyamide foam material is prepared by treating polyamide with concentrated solution of zinc chloride in HCl to make a gum-like solid and supernatant liquid, admixing 1,3,5trioxane and the gum-like solid to make a homogeneous, light, and flowable paste which is then cured in a suitable mold at atmospheric conditions, preferably in the presence of weakly acidic water. The membranes are useful, e.g., for bandages and the nylon foam is useful, e.g., as a construction material, for trays, containers, etc.

ilnite States Patent 1191 Sklar 14 1 Jan.l0,]1973 1541 METnon OFPRODUCING POLYAMIDE MEMBRANES AND mom FOAMED PRODUCTS [76] Inventor:Lawrence A. Sklar, 14697 Northeast 18th Avenue, North Miami, Fla. 3316122 Filed: DEC. 31, 1969 211 Appl. No.: 889,331

Related U.S. Application Data [63] Continuation-impart of Ser. No.560,124, June 24,

1966, abandoned.

[58] Field of Search 264/41 49,289, Dig. 61, Dig. 62,

264/288; 260/25 M, 78 R, 78 A, 2.5 N, 72 N [56] I References CitedUNITED STATES PATENTS 3/1957 Lovell et al., ..264/41 UX 3,427,179 2/1969Da'vis- ..264/4l M X Primary ExaminerPhilip E. Anderson Att0rneyWayne C.Jaeschke [57] 1 ABSTRACT Polyamide membranes of desired thickness areprepared by dissolving polyamide in HCl, admixing paraformaldehyde andthen zinc chloride to produce a viscous paste, forming expandedgelatinous material by treatment with formaldehyde, curing this materialand applying tensile stress thereto. Rigid, water insoluble, polyamidefoam material is prepared by treating polyamide with concentratedsolution of zinc chloride in HCl to make a gumlike solid and supernatantliquid, admixing 1,3,5-trioxane and the gum-like solid to make ahomogeneous, light, and flowable paste which is then cured in a suitablemold at atmospheric conditions, preferably in the presence of weaklyacidic water. The membranes are useful, e.g., for bandages and the nylonfoam is useful, e.g., as a construction material, fortrays, containers,etc.

9 Claims, 2 Drawings METHOD OF PRODUCING POLYAMIDE MEMBRANES AND RIGIDFOAMED PRODUCTS This application is a continuation-in-part of my priorand co-pending application Ser. No. 560,124, filed June 24, 1966 and nowabandoned.

The present invention relates to the production of novel polyamideproducts by new and useful methods; More specifically, in one aspect,the present invention relates to the production of unsupported,relatively thin, flexible, and vapor permeable sheets or membranes ofpolyamide material. In another aspect, the invention relates to theproduction of rigid, insoluble, polyamide foam materials.

A great variety of processes are known for the formation of usefulproducts from nylon material. It is, of course, well known that nylon66, for example, can be extruded from a melt into monofilaments or spunfrom a solution thereof in formic acid or phenol. When the filaments arecold drawn to about four times their original length, the molecules areoriented along the axis of the fiber resulting in fibers which areelastic and have a higher tensile strength than silk either in a dry orwet condition. By reason of the exceptional commercial success enjoyedby nylon filaments large and highly respected research organizationshave expended extraordinary sums in time and money in the exploration ofpossible routes to obtain other semifinished as well as finished nylonproducts. Included in such explora tion has been considerable efforts torealize in an economically practicable manner, relatively thin, i.e.,between l/l6 inch and 1/ 10,000 inch, nylon sheets of high tensilestrength. Unfortunately, the objectives of such efforts are notcompletely fulfilled in the prior art.

Not withstanding the foregoing, I have now discovered a novel andunobvious process for preparing relatively thin, flexible, polyamidemembranes having high tensile strength, and, of essential importance forthe novel uses which I have found for the product of this process,appreciable permeability with respect to fluids including both gaseousand liquid materials. That is to say, the polyamide membranes preparedin accordance with my novel process have a measurable and controllablerate of diffusion of gaseous material as well as liquid material,bi-directionally, through the intact membrane which can be expressed interms of unit volume of fluid passing through a given unit of area ofmembrane surface per unit time. The rate of diffusion of gaseousmaterial such as, e.g., oxygen, is, quite unexpectedly, substantiallygreater than the rate of diffusion of liquid material through the intactmembrane making such membranes useful as means for diffusing gaseousmaterial which can be retained on one side of a membrane into a liquidretained on the membrane surface, itself. In this regard the membranesof the present invention are shown to be useful in the controlledoxygenation of blood. The membranes of the present invention are shownto be useful as bandage material in covering wounds, particularly, ascoverage for burns. Medication can be diffused inwardly through theinert membrane material while the gaseous products of biologicaldecomposition diffuse outwardly away from the wound. Surprisingly, thefluid permeable material prepared in accordance with the process of thepresent invention appears from unaided visual observation thereof to bea thin, continuous film of the polyamide material, which in the case ofcertain relatively thin sheets which I have prepared, are completelytransparent. The polyamide membranes of this invention can be preparedhaving a thickness between about onesixteenth and about oneten-thousandth of an inch as desired. Such membranes can be preparedhaving superficial pores disposed on the surfaces thereof which impart asponge-like appearance as well as resilience to the membrane. Ifdesired, the membranes can likewise be prepared having a surface whichappears to be perfectly smooth and regular by unaided visualobservation. In general, however, the surface condition of the membranewhich may be desired for the specific applications thereof does notalter the essential properties of the membranes.

Generally, the aforesaid membranes are prepared in accordance with theprocess of the present invention by dissolving a suitable polyamidematerial in hydrochloric acid solution, admixing solidformaldehyde-containing-material and then zinc chloride, in that order,with the resulting polyamide-acid solution in such proportions that aviscous paste is obtained. Formaldehyde solution is then contacted withthe viscous paste for an extended period of time uner conditions whichcause the formaldehyde to be absorbed in the viscous paste causing sameto swell and to become an expanded, gelatinous material. Sufficientcontact time between the gelatinous material and formaldehyde ispermitted, preferably, such that maximum formaldehyde absorption occurs.The gel is then separated from any liquid formaldehyde which may bepresent, and at this point exhibits the properties of a viscous fluid.The gel is permitted to cure, preferably under atmospheric conditions,until such time as it exhibits a concurrence of properties whichincludes plastic adherence and resilience at which time it is workedinto the membrane of the present invention by the application of tensilestress which causes elongation and essentially uniform reduction inthickness thereof.

I have also discovered that by varying the procedure herein beforedescribed for making the novel membranes, I can produce a new and usefulrigid, polyamide foam material. The novel, rigid foam is substantiallyless dense than extruded or spun polyamides. However, the product foamis insuluble in water and common solvents and possesses great strengthand shatter resistance. In accordance with the new and surprising methodof preparation, the rigid polyamide foam can be molded under atmosphericconditions into any desired shape. For example, the rigid, polyamidefoam is useful for construction materials, e.g., wall and ceiling panelsand floor tiles. Additionally, the rigid foam material is capable ofbeing sawed, drilled and sanded. Nails and screws can be permanentlyinserted into the material without splitting or damaging the material inany way. The nylon foam can also be used to make trays, containers ofall typed, as packaging material, gasket material and the like.

Generally, the rigid polyamide foam is prepared in the following manner.A suitable polyamide is contacted with an acidic, concentrated solutionof zinc chloride. The mixture is stirred until the polyamide isconverted to a gum-like material covered by supernatant acid, zincchloride solution. The gum-like material is then treated by adding solidformaldehyde-containing material until the gum swells and absorbs thesupernatant liquid. At this point the material is homogeneous and hasthe consistency of a light, flowable paste. In order to achieve a light,flowable paste of optimum consistency, about one part by weight of solidformaldehyde-containing material is added for every part of polyamide.This ratio is not critical however. The light, flowable paste is pouredinto a suitable mold or form, (which should be acid resistant). Therigid, polyamide foam of this invention can be prepared from the abovedescribed paste simply by permitting it to set up in the form underatmospheric conditions. Preferably, in order to avoid formation of anundesirable skin on the surface of the final product, the paste iscovered with weakly acidified water during the curing period which maytake from 1 to 2 hours. When the'rigid foam has set up, the product isleached with cold water to remove residual chemicals. The resultingrigid foam has the properties herein before described.

In order to achieve a rigid polyamide foam of desired density, zincchloride: HCl: polyamide weight ratios between about l:2:0.5 and about1:8:4 are preferred. Increasing the relative amount of polyamide makes afoam product of greater density; and, correspondingly, decreasing thepolyamide lowers the density of the foam product.

Reference is made to the two figures of the drawing which illustrate twopreferred embodiments of the process of the present invention.

FIG. 1 illustrates the process for making polyamide membranes.

FIG. 2 illustrates the process for making rigid, polyamide foam.

The polyamide starting materials useful in the presentinvention are wellknown in the literature and are described in detail for example in US.Pat. Nos. 2,071,250, 2,07l,253, and 2,130,948. These polymers arecrystalline rather than resinous and are linear, long chain products ofhigh molecular weight. The polyamides employed in the present inventioninclude in addition to polyamides obtained solely from polyamide formingreactants, linear polymers containing recurring amide groups as anintegral part of the main chain of atoms in the molecule, such as, e.g.,the ester-amide interpolymers obtained from the. inclusion of otherbifunctional reactants with the polyamide-forming reactants. Uponhydrolysis with mineral acids the polyamides yield monomericamide-forming reactants. For example, a polyamide derived from a diamineand a dibasic carboxylic acid yields upon hydrolysis thereof withhydochloric acid, the dibasic carboxylic acid and the diaminehydrochloride. Polyamide's having an intrinsic viscosity, as defined inUS. Pat. No. 2,130,948, of at least about 0.4 and preferably betweenabout 0.5 and about 2.0 are most useful.

As the initial step in the process of making the novel membranes asuitable polyamide, preferably prepared in cominuted form, is contactedwith sufficient hydrochloric acid solution to effect substantiallycomplete dissolution thereof. For this purpose polyamide to hydrochloricacid ratios between about 0.1 and about 1 to l have been found useful.After the solution has been prepared solidformaldehyde-containingmaterial selected from the group consisting ofthe cyclic trimer, l, 3, S-trioxane, which isa crystalline material, andparaformaldehyde is admixed therewith. The addition of the solidformaldehyde at this point is essential in order to prevent degradationof the polyamide to lower molecular weight material. Such materialadditionally appears to improve the formation of the gelatinous materialhereinafter described. Ratios of solid formaldehyde containing materialto polyamide which have been found useful at this point in the processare between about 0.1 to l and about 0.5 to 1.

Following the introduction of the formaldehyde, sufficient zinc chlorideis stirred into the resulting mixture until a smooth, viscous paste isobtained. It has been found that the specific reactant, namely, zincchloride is required in order to produce a gelatinous materialhereinafter described having suitable drawing properties for the purposeof making membranes in accordance with the present invention. It haslikewise been found that the use of an insufficient amount of zincchloride results in the preparation of gelatinous material havingundesirable drawing properties. Therefore, zinc chloride to polyamideratios of at least about 0.1 to l and preferably above about 0.5 to lare employed.

When a viscous paste has been prepared, it is than brought into contactwith formaldehyde solution for a period of time sufficient to permitsubstantial saturation thereof by reason of absorption of theformaldahyde therein resulting in the production of an expandedgelatinous material. A preferable method of ad ding the formaldehydesolution to the viscous paste in order to produce the loose, workablegel useful for the purposes of the present invention comprisesintroducing the formaldehyde to the viscous paste, which may becontained in a suitable reaction vessel, over a protracted period oftime while providing suitable stirring or agitation in order to permitabsorption or soaking up of the formaldehyde in the interstices of thepaste which appears to swell during the absorption period. Where theformaldehyde is added too rapidly or if insufficient contact time isprovided a hard, unworkable gel is obtained. In accordance with thepreferred method of addition, at a certain point which is readilyobservable, the gelatinous phase reaches its maximum capacity forformaldehyde retention and separation of a liquid formaldehyde phase isobserved. Formaldehyde addition can be terminated at this point and thenbe separated from any supernatant liquid.

The gel produced in accordance with the above described procedureexhibits flow properties like those of a viscous fluid. That is to say,there appears to be little if any bonding between the particlescomprising the mass and the rate of flow of the mass is proportional tothe applied stress. The gel at this point will flow to some extent underits own weight, for example, if it is heaped as a mass on a smoothsurface. The gel at this point is not yet suitable for working into themembranes of the present invention. It has been found that it isessential to cure the gel, e.g., by permitting it to stand underatmospheric conditions for a period of time until the gel undergoes areadily observable change from a viscous fluid to a mass exhibitingplastic properties. Under atmospheric conditions the curing time whichis required usually varies between about one-half and about 1% hours. Itis contemplated that the curing time can be reduced by the applicationof temperatures above cured contraction of the gel occurs and thematerial exhibits elasticity or resilience to a minor degree and plasticadherence is apparent. That is to say, in one aspect the mass behaveslike modeling clay in that if a portion of the material is broken offfrom the remainder it can be reunited therewith by pressing it back intothe mass. The material when deformed to a minor extent, e.g., topercent, will regain its original shape. It is essential that when theproperties of plastic adherence and resilience concur in the mass, ascan be readily demonstrated, for example, by hand testing the materialin the above described manner, that the membranes be drawn therefromduring this period. Otherwise, the material upon additional curing losesits plastic properties becoming hard and unworkable. Thus, closeobservation and periodic testing of the mass is required during thecuring operation in order to insure drawing of the membranes at theappropriate time.

It is essential that the gel cured in the above described manner bedrawn into membranes by the application of tensile stress thereto. Ithas been found that the gel resists rolling and extruding methods. Theapplication of compressive force to the gel actually causes it to becomehard and unworkable. Therefore, in order to draw the membrane of thepresent invention tensile stress is applied to the mass either by handor suitable machine and in either an intermittent or continuous fashionby engaging peripheral regions of the mass and applying force in adirection radially outward with respect to the center of the mass. Thebest results are obtained when the application of force is essentiallyuniform over the cross-sectional area of the mass. The

uniform application of force to the mass can be accomplished by the useof a suitable rack or frame over which the membranes are disposed duringapplication of the tensile stress causing elongation and uniformreduction in thickness thereof.

Subsequent to the formation of the membrane in the above describedmanner the acidic medium isremoved therefrom by any suitable leachingprocedure such as, e.g., leaching the membrane with relatively coldwater, i.e., 40 60F, over a protracted period of time. It has been foundthat relatively slow, cold water leaching of the membranes is preferablefor the preparation of membranes to be employed in certain surgicalapplication hereinafter described. Another method of removing the acidicmedium comprises treating the membranes with a base such as sodiumhydroxide and triethanol amine, for example, followed by souring with anacid and then water washing to remove the salts formed during theneutralization reaction. Such treatment yields a relatively smoothsurface on each side of the membrane and the membranes produced therebyexhibit high tensile strength. Another method which can be employedwhere it is desirable to cause the formation of superficial poresresulting in a sponge like appearance of the surfaces of the finalproduct comprises treatment with sodium bicarbonate, for example. Thecarbon dioxide which is released causes the above described poreformation and results in the preparation of membranes havingexceptionally low effective thickness.

Having thus described the invention in general terms reference is nowmade to several specific examples of the above described procedure inorder to provide a better understanding of the present inventionalthough it is to be understood that the invention should not beconstrued as being unduly limited thereby.

EXAMPLE 1 interpolyamide in the amount of 400 parts prepared fromhexamethylene diammonium adipate and caprolactam in a ratio of 60:40,which had been ground to pass a 14 mesh screen, was dissolved in 600parts of 38% HCl. parts of 1,3,5 trioxane powder was added and stirredin. 200 parts of zinc chloride in granular form was then added and themass was thoroughly agitated until a smooth past was formed. Liquidformaldehyde solution in a total amount of 1,300 parts was then addedand stirred into the paste over a period of about one-half hour. Theexpanded gelatinous material which resulted was separated from liquidformaldehyde and allowed to set under atmospheric conditions for aboutone hour at which time the material appeared to be in suitable conditionfor working. Tension was applied to the mass, by hand, in as nearly auniform manner as possible and when the material was sufficientlyattenuated, the formation of the membrane was completed employing asmall frame for the purpose of supporting the material being worked.

In the above described manner a thin, continuous film or membrane ofpolyamide material having the properties hereinbefore described andhaving a thickness of about one-thousandth inch was prepared. Residualchemical materials were removed from the membrane by leaching in coldwater. The product was transparent.

The preferential vapor permeability of a membrane can be determined in asimple manner by measuring the change in PH of water retained on thecupped surface of known area of a membrane, when such membrane issituated in covering relationship over a container of aqueous ammonia ofknown concentration.

EXAMPLE 2 This example illustrates the preparation of rigid polyamidefoam structure by another embodiment of the process of this invention.

One part zinc chloride in granular form was stirred into one part waterto form a highly concentrated solution. The solution was permitted tocool down to about room temperature. Four parts of 38% HCl were thenadded. Two parts interpolyamide (same as example 1) were then stirredinto the acid zinc chloride solution. The mixture forms a gum-likematerial with a supernatant liquid comprising acid zinc chloride. 1, 3,5 trioxane powder (2 parts) was added with mixing until the gum becameswollen and absorbed all of the supernatant liquid. After addition ofthe trioxane was completed, the gum had the consistency of a smoothlight and flowable paste. The paste was poured into a shallow acidresistant pan which was used as a mold for the final product. A weak (2percent) aqueous solution of HCl was slowly poured over the top of thepaste until it was completely covered. The mold was allowed to standabout two hours. The product was then removed from the pan, leached withcold water and dried.

The molded product had a rigid polyamide foam structure. The product wasinsoluble in water although it was capable of being wetted by andabsorbing small quantities of water. The polyamide foam is substantiallylighter (less dense) than conventional nylon. The foam can be cut bymeans of a saw and nails can be pounded into the foam without splittingit. On the other hand the foam is so tough and shatter resistant thatrepeated hammering failed to shatter or make substantial depressions inthe foam.

The surface of the foam has a smooth surface conforming to the mold. Asoft, velvet-like finish can be imparted to the polyamide foam byabrading the material with an emery stone or sandpaper.

Having thus described the invention, many modifications and alterationsthereof will become apparent to those skilled in the art withoutdeparting from the spirit and scope thereof.

What is claimed is:

1. A process of preparing a polyamide membrane which comprises:preparing a solution of a crystalline linear and long chain highmolecular weight polyamide in hydrochloric acid, admixing solidformaldehydecontaining material selected from the group consisting of 1,3, trioxane and paraformaldehyde and said solution in an amountsufficient to prevent substantial degradation thereof, admixing zincchloride with the resulting solution until a viscous paste is obtained,contacting said viscous paste with formaldehyde solution for a period oftime sufficient to form an expanded polyamide-containing gelatinousmaterial, separating said gelatinous material from formaldehydesolution, curing said gelatinous material for a period of timesufficient to produce therefrom a material exhibiting propertiesincluding plastic adherence and resilience, and during the period ofconcurrence of such properties in said material elongating same by theapplication of essentially uniform tensile stress thereto to producerelatively thin, continuous, flexible, and vapor permeable polyamidemembrane of a desired thickness, and removing residual chemicals fromsaid membrane.

2. The process of claim 1 in which the polyamide to hydrochloric acidratio is between about 0.l:l and about 1:1.

3. The process of claim 1 in which the ratio of solidformaldehyde-containing material to polyamide is between about 0.1 :1and about 0.5: l.

4. The process of claim 1 in which the zinc chloride to polyamide ratiois at least about 0.1:1.

5. The process of claim 1 in which said liquid formaldehyde is added tosaid viscous paste slowly with agitation over a period of time tosaturate the resulting gelatinous material.

6. The process of claim 1 in which said gelatinous material is curedunder essentially atmospheric conditions for a period of time betweenabout 9% and about 1% hours.

7. The process of claim 1 in which residual chemicals remaining in saidmembrane are extracted therefrom by leaching thereof in relatively coldwater.

8. A process of preparing a rigid water insoluble polyamide foam whichcomprises: admixing a crystalline linear and long chain high molecularweight polyamide and a concentrated solution of zinc chloride inhydrochloric acid to form a gum-like solid material and a supematantliquid, admixing solid formaldehyde-contaming material selected from thegroup consisting of l, 3, 5 trioxane and paraformaldehyde and saidgumlike solid material to form a homogeneous, light and flowable paste,curing said paste in a suitable molding zone thereby forming rigid,water insoluble, polyamide foam of desired shape and size.

9. The process of claim 8 in which said molding is maintained underessentially atmospheric conditions, and said paste is maintained underweakly acidic water.

2. The process of claim 1 in which the polyamide to hydrochloric acidratio is between about 0.1:1 and about 1:1.
 3. The process of claim 1 inwhich the ratio of solid formaldehyde-containing material to polyamideis between about 0.1:1 and about 0.5:1.
 4. The process of claim 1 inwhich the zinc chloride to polyamide ratio is at least about 0.1:1. 5.The process of claim 1 in which said liquid formaldehyde is added tosaid viscous paste slowly with agitation over a period of time tosaturate the resulting gelatinous material.
 6. The process of claim 1 inwhich said gelatinous material is cured under essentially atmosphericconditions for a period of time between about 1/2 and about 1 1/2 hours.7. The process of claim 1 in which residual chemicals remaining in saidmembrane are extracted therefrom by leaching thereof in relatively coldwater.
 8. A process of preparing a rigid water insoluble polyamide foamwhich comprises: admixing a crystalline linear and long chain highmolecular weight polyamide and a concentrated solution of zinc chloridein hydrochloric acid to form a gum-like solid material and a supernatantliquid, admixing solid formaldehyde-containing material selected fromthe group consisting of 1, 3, 5 trioxane and paraformaldehyde and saidgum-like solid material to form a homogeneous, light and flowable paste,curing said paste in a suitable molding zone thereby forming rigid,water insoluble, polyamide foam of desired shape and size.
 9. Theprocess of claim 8 in which said molding is maintained under essentiallyatmospheric conditions, and said paste is maintained under weakly acidicwater.